Process for producing hexafluoroethane and use thereof

ABSTRACT

The present invention intends to provide a process for producing CF 3 CF 3  with good profitability using CF 3 CHF 2  containing a compound having chlorine atom within the molecule, and use thereof. 
     In the process of the present invention, a gas mixture containing CF 3 CHF 2  and a compound having chlorine atom within the molecule is reacted with hydrogen fluoride in the presence of a fluorination catalyst, thereby converting CClF 2 CF 3  as a main impurity into CF 3 CF 3 , and CF 3 CHF 2  containing CF 3 CF 3  is reacted with fluorine gas in the gaseous phase in the presence of a diluting gas.

CROSS REFERENCE TO RELATED APPLICATION

This application is a 371 of PCT/JP01/05256 filed Jun. 20, 2001, which claims benefit of an application filed under 35 U.S.C. §111(a) claiming benefit pursuant to 35 U.S.C. §119(e)(1) of the filing date of Provisional Application No. 60/230,806 filed on Sep. 7, 2000, pursuant to 35 U.S.C. §111(b).

DETAILED DESCRIPTION OF THE INVENTION

1. Technical Field of the Invention

The present invention relates to a process for producing hexafluoroethane, comprising a step of reacting a gas mixture containing pentafluoroethane and a compound having chlorine atom with hydrogen fluoride in the gaseous phase in the presence of a fluorination catalyst to fluorinate the compound having chlorine atom and a step of reacting the gas mixture containing pentafluoroethane and the fluorinated compound with fluorine gas in the gaseous phase in the presence of a diluting gas, and also relates to the use thereof.

2. Background Art

Pentafluoroethane (hereinafter referred to as “CF₃CHF₂”) is used, for example, as a refrigerant for low-temperature use or a starting material for the production of hexafluoroethane (hereinafter referred to as “CF₃CF₃”).

For the production of CF₃CHF₂, for example, the following methods have been heretofore known:

(1) a method of fluorinating perchloroethylene (CCl₂═CCl₂) or a fluoride thereof with hydrogen fluoride (see, JP-A-5-97724 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”), JP-A-6-506221, JP-A-7-76534, JP-A-7-118182, JP-A-8-268932 and JP-A-9-511515),

(2) a method of performing hydrogenolysis of chloropentafluoroethane (CClF₂CF₃), and

(3) a method of reacting a fluorine gas with a halogen-containing ethylene (see, JP-A-1-38034).

When these methods for producing CF₃CHF₂ are used, the objective CF₃CHF₂ contains a compound having chlorine atom within the molecule as main impurities. The compound having chlorine atom within the molecule includes a compound having one carbon atom within the molecule, such as chloromethane, chlorodifluoromethane and chlorotrifluoromethane, a compound having two carbon atoms within the molecule, such as chloropentafluoroethane, dichlorotetrafluoroethane, chlorotetrafluoroethane and chlorotrifluoroethane, and an unsaturated compound such as chlorotrifluoroethylene.

In the case of producing CF₃CF₃ by a direct fluorination reaction of reacting CF₃CHF₂ with a fluorine gas (F₂), if CF₃CHF₂ contains the compound having chlorine atom within the molecule, chlorine, hydrogen chloride, chlorine fluoride or different kinds of chlorofluorocarbons are generated in the reaction with fluorine gas. Even when hydrofluorocarbons (HFC) or perfluorocarbons (PFC) are contained in CF₃CHF₂, there arises no particular problem, however, for example, chloromethane (CH₃Cl) or chlorodifluoromethane (CHClF₂) reacts with fluorine gas to produce chlorotrifluoromethane (CClF₃). The objective CF₃CF₃ and chlorotrifluoromethane form an azeotropic composition, therefore, CClF₃ is difficult to remove even by performing distillation, adsorption for purification, or the like. Accordingly, in the case of reacting CF₃CHF₂ with a fluorine gas to produce CF₃CF₃, the amount of the compound having chlorine atom within the molecule contained in CF₃CHF₂ should be reduced as much as possible.

According to conventional production methods for CF₃CHF₂, the total amount of the compound having chlorine atom within the molecule is sometimes as high as about 1 vol %. Therefore, a distillation operation is repeated for removing these compounds contained in CF₃CHF₂ and elevating the purity of CF₃CHF₂, however, this has such a problem that the distillation cost increases, the distillation loss is caused, the profitability is bad and some compounds having chlorine atom within the molecule form an azeotropic mixture or an azeotrope-like mixture with CF₃CHF₂ and are very difficult to remove only by the distillation operation. In particular, chloropentafluoroethane (hereinafter referred to as “CClF₂CF₃”) is usually contained in CF₃CHF₂ in a concentration of thousands of ppm or more but since an azeotropic mixture is formed by CF₃CHF₂ and CClF₂CF₃, the separation is hardly attained by distillation which is a commonly used separation and purification method.

For separating CClF₂CF₃ contained in CF₃CHF₂, various methods have been proposed, for example,

(1) a method of adding a third component to a mixture of CF₃CHF₂ and CClF₂CF₃ and performing the extractive distillation (see, JP-A-6-510980, JP-A-7-133240, JP-A-7-258123, JP-A-8-3082, JP-A-8-143486 and JP-A-10-513190),

(2) a method of removing CClF₂CF₃ contained in CF₃CHF₂ using an adsorbent (see, JP-A-6-92879 and JP-W-8-508479 (the term “JP-W” as used herein means an “unexamined published international patent application”)), and

(3) a method of converting CClF₂CF₃ contained in CF₃CHF₂ into CF₃CHF₂ in the presence of a hydrogenation catalyst (see, JP-A-7-509238, JP-A-8-40949, JP-A-8 -301801 and JP-A-10-87525).

However, these methods have a problem, that is, the method of (1) requires a step of recovering the third component from the mixture of CClF₂CF₃ and the third component, the method of (2) requires a step of regenerating the adsorbent, and the method of (3) suffers from reduction in the catalytic life due to hydrogen chloride produced.

Problems to be Solved by the Invention

The present invention has been made under these circumstances and the object of the present invention is to provide a method for producing CF₃CF₃ with good profitability using a gas mixture containing CF₃CHF₂ and a compound having chlorine atom within the molecule in the method for producing CF₃CF₃ which is used as an etching or cleaning gas in the process of producing a semiconductor device, and also provide a use thereof.

Means to Solve the Problems

As a result of extensive investigations to solve the above-described problems, the present inventors have found that in the method for producing CF₃CF₃, when a gas mixture containing CF₃CHF₂ and a compound having chlorine atom within the molecule as impurities is reacted with hydrogen fluoride in the presence of a fluorination catalyst to convert CClF₂CF₃ which is contained in the gas mixture, into CF₃CF₃ and then performing a direct fluorination reaction of reacting the resulting gas mixture containing CF₃CHF₂ and CF₃CF₃ with a fluorine gas in the gaseous phase in the presence of a diluting gas, the above-described problems can be solved. The present invention has been accomplished based on this finding. The present invention provides a process for producing CF₃CF₃ and use thereof, described in [1] to [19] below.

[1] A process for producing hexafluoroethane, comprising the following two steps:

(1) a step of reacting a gas mixture containing pentafluoroethane and a compound having chlorine atom with hydrogen fluoride in the gaseous phase in the presence of a fluorination catalyst to fluorinate the compound having chlorine atom; and

(2) a step of reacting the gas mixture containing pentafluoroethane and the fluorinated compound obtained in the step (1) with a fluorine gas in the gaseous phase in the presence of a diluting gas.

[2] The process for producing hexafluoroethane as described in [1], wherein the compound having chlorine atom is at least one compound selected from the group consisting of chloromethane, chlorotrifluoromethane, chloropentafluoroethane, dichlorotetrafluoroethane, chlorotetrafluoroethane, chlorotrifluoroethane and chlorotrifluoroethylene.

[3] The process for producing hexafluoroethane as described in [1] or [2], wherein the total amount of the compound having chlorine atom contained in the gas mixture of the step (1) is 1 vol % or less.

[4] The process for producing hexafluoroethane as described in [1] or [2], wherein the total amount of the compound having chlorine atom contained in the gas mixture of the step (1) is 0.5 vol % or less.

[5] The process for producing hexafluoroethane as described in any one of [1j to [4], wherein in the step (1), the fluorination catalyst is a bulk catalyst obtained by adding indium to an oxide of chromium.

[6] The process for producing hexafluoroethane as described in any one of [1] to [5], wherein in the step (1), the temperature at the reaction with hydrogen fluoride in the presence of a fluorination catalyst is in the range of 150 to 480° C.

[7] The process for producing hexafluoroethane as described in any one of [1] to [6], wherein in the step (1), the molar ratio of hydrogen fluoride/organic substance contained in the gas mixture is in the range of 0.5 to 5.

[8] The process for producing hexafluoroethane as described in any one of [1] to [7], wherein a step of removing an acid content containing hydrogen chloride produced is conducted before the step (2).

[9] The process for producing hexafluoroethane as described in any one of [1] to [8], wherein a step of separating chlorotetrafluoroethane and/or chlorotrifluoroethane, and returning the chlorotetrafluoroethane and/or chlorotrifluoroethane separated to the step (1) is conducted before the step (2).

[10] The process for producing hexafluoroethane as described in any one of [1] to [9], wherein in the step (2), the total amount of the compound having chlorine atom contained in the gas mixture is 0.02 vol % or less.

[11] The process for producing hexafluoroethane as described in any one of [1] to [10], wherein in the step (2), the fluorinated compound contained in the gas mixture is mainly composed of hexafluoroethane.

[12] The process for producing hexafluoroethane as described in any one of [1] to [11], wherein in the step (2), the diluting gas is a gas containing at least one selected from the group consisting of tetrafluoromethane, hexafluoroethane, octafluoropropane and hydrogen fluoride.

[13] The process for producing hexafluoroethane as described in any one of [1] to [12], wherein in the step (2), the diluting gas is a gas rich in hydrogen fluoride.

[14] The process for producing hexafluoroethane as described in any one of [1] to [13], wherein in the step (2), the temperature at the reaction of gas mixture containing the fluorinated compound with fluorine gas is in the range of 250 to 500° C.

[15] The process for producing hexafluoroethane as described in any one of [1] to [14], wherein in the step (2), the temperature at the reaction of gas mixture containing the fluorinated compound with fluorine gas is in the range of 350 to 450° C.

[16] A hexafluoroethane product comprising hexafluoroethane having a purity of 99.9997 vol % or more.

[17] The hexafluoroethane product as described in [16], wherein the content of the compound having chlorine atom is 1 volppm or less and the content of the pentafluoroethane is 1 volppm or less.

[18] An etching gas comprising the hexafluoroethane product described in [16] or [17].

[19] A cleaning gas comprising the hexafluoroethane product described in [16] or [17].

In summary, the present invention provides “a process for producing CF₃CF₃, comprising a step of reacting a gas mixture containing CF₃CHF₂ and a compound having chlorine atom with hydrogen fluoride in the gaseous phase in the presence of a fluorination catalyst to fluorinate the compound having chlorine atom and a step of reacting a gas mixture containing CF₃CHF₂ and the fluorinated compound obtained by the above-described step with a fluorine gas in the gaseous phase in the presence of a diluting gas”, “an CF₃CF₃ product comprising CF₃CF₃ having a purity of 99.9997 vol % or more”, “an etching gas comprising the above-described CF₃CF₃ product” and “a cleaning gas comprising the above-described CF₃CF₃ product”.

MODE FOR CARRY OUT THE INVENTION

The production process for CF₃CF₃ and use thereof according to the present invention are described in detail below.

As described above, CF₃CHF₂ for use in the present invention is generally produced by fluorinating perchloroethylene (CCl₂═CCl₂) or a fluoride thereof with hydrogen fluoride (HF), and CF₃CHF₂ contains a compound having chlorine atom derived from the starting material, such as chloromethane, chlorodifluoromethane, chlorotrifluoromethane, chloropentafluoroethane, dichlorotetrafluoroethane, chlorotetrafluoroethane and chlorotrifluoroethane. In order to purify CF₃CHF₂ containing these compounds to a high purity, known methods by a distillation operation are employed, however, these methods have such a problem that these are not economical since the compound and CF₃CHF₂ form an azeotropic mixture or an azeotrope-like mixture, the purification by separation is very difficult, the number of stages of the distillation tower or the number of the distillation towers must be increased, and the cost for equipment or energy increases.

In the present invention, the compound having chlorine atom contained in CF₃CHF₂ as impurities is fluorinated with hydrogen fluoride at an elevated temperature in the presence of a fluorination catalyst and thereby converted into hydrofluorocarbon (HFC) or perfluorocarbon (PFC). For example, in fluorinating CClF₂CF₃ or chlorotetrafluoroethane contained as impurities in CF₃CHF₂ using hydrogen fluoride, a reaction shown by the following formula (1) or (2) takes place:

CF₃CClF₂+HF→CF₃CF₃+HCl  (1)

CF₃CHClF+HF→CF₃CHF₂+HCl  (2)

The product is HFC or PFC free of chlorine atom, and hydrogen chloride is produced as a by-product.

In the present specification, the gas mixture containing CF₃CHF₂ and the compound having chlorine atom is sometimes referred to as “starting gas mixture”.

In this fluorination reaction, the compound which is converted into HFC or PFC is chloromethane, chlorodifluoromethane, chlorotrifluoromethane, chloropentafluoroethane, dichlorotetrafluoroethane, chlorotetrafluoroethane and chlorotrifluoroethane. These compounds are usually contained in CF₃CHF₂ in a total amount of thousands of ppm or more. When the starting gas mixture containing these compounds is reacted with a fluorine gas, the methane-type compounds are converted into CClF₃ and the ethane-type compounds are converted into CClF₂CF₃, therefore, CF₃CF₃ obtained after the reaction contains CClF₃ and CClF₂CF₃ as main impurities.

CClF₂CF₃ scarcely reacts with a fluorine gas at low temperatures. However, according to the investigations by the present inventors, for example, at a reaction temperature of 400° C., the amount of CClF₃ produced by the decomposition of CClF₂CF₃ is 1 ppm or less when the concentration of CClF₂CF₃ contained in the starting gas mixture is about 800 ppm or less, and about 2 ppm of CClF₃ is produced when the concentration of CClF₂CF₃ exceeds about 2,000 ppm. CClF₃ forms an azeotropic mixture with CF₃CF₃, therefore, even if the concentration is low, this compound is difficult to remove by an operation of distillation, adsorption for purification or the like. Accordingly, it is preferred that not only a compound which produces CClF₃ upon reaction with a fluorine gas is removed from CF₃CHF₂ as a starting material but also the CClF₂CF₃ content is reduced to a low concentration as much as possible.

The total amount of the compound having chlorine atom contained in the starting gas mixture for use in the present invention is preferably 1 vol % or less, more preferably 0.5 vol % or less, still more preferably 0.3 vol % or less. If the concentration of the compound having chlorine atom exceeds 1 vol %, the reaction must be performed at a high temperature and the life of the fluorination catalyst is disadvantageously shortened, moreover, a side reaction proceeds at the same time and the productivity decreases.

The fluorination catalyst comprises at least one element selected from the group consisting of chromium, nickel, zinc, indium and garium, and may be a known catalyst such as supported catalyst or bulk catalyst.

In the case of the supported catalyst, carrier is preferably an alumina and/or partially fluorinated alumina, and supporting ratio is preferably 30 wt % or less. In the case of the bulk catalyst, particularly preferred is those containing chromium as main component, and having atomic ratio of nickel, zinc, indium and/or garium to chromium of 0.01 to 0.6. In the present invention, most preferred is a bulk catalyst obtained by adding indium to an oxide of chromium.

In the step of fluorinating the compound having chlorine atom, the reaction temperature is preferably from 150 to 480° C. If the reaction temperature exceeds 480° C., the reaction is adversely affected, for example, the catalyst deteriorates or a side reaction proceeds, and this is not preferred. Although it may vary depending on the concentration of the compound contained in the starting gas mixture, a preferred reaction temperature can be selected according to the kind of the compound. For example, in the reaction of CClF₂CF₃ shown in formula (1), the reaction temperature is preferably 400° C. or more, and in the reaction of CF₃CHClF shown by formula (2), the reaction temperature is preferably 300° C. or more.

In the case of a reaction of chlorodifluoromethane (CHClF₂) with hydrogen fluoride, a reaction shown by the following formula (3) takes place:

CHClF₂+HF→CHF₃+HCl  (3)

In this reaction, the reaction temperature is preferably 150° C. or more and if the reaction temperature exceeds 400° C. or more, a reverse reaction disadvantageously proceeds.

In the step of fluorinating a compound having chlorine atom, the reaction temperature sometimes varies depending on the kind of the compound as described above. Accordingly, in the case where a plurality of compounds are contained and these are different from each other in the optimal reaction temperature region or the concentration of each compound is high, two or more units of reactors are preferably used, though one unit of a reactor is usually sufficient.

The amount of HF used is, in terms of the molar ratio to the organic substance of the starting gas mixture containing CF₃CHF₂ (HF/organic substance), suitably from 0.5 to 5, preferably from 0.5 to 2. If the molar ratio is less than 0.5, the reaction is hard to proceed, whereas if it exceeds 5, a large reactor is necessary and this is not profitable.

Furthermore, in the step of fluorinating a compound having chlorine atom, the reaction pressure is preferably from atmospheric pressure to 1.5 MPa. If it exceeds 1.5 MPa, the apparatus is disadvantageously required to have pressure resistance or the like.

In the present invention, the reaction with hydrogen fluoride is performed in the presence of a fluorination catalyst using the above-described reaction conditions, and then CF₃CHF₂, chlorine atom-free impurities mainly comprising HFC or PFC, and hydrogen chloride as a by-product are contained in the reaction product. In the case of CF₃CHF₂, as the reaction temperature becomes higher, a side reaction with hydrogen chloride more proceeds as shown in the following formula (4):

CF₃CHF₂+HCl→CF₃CHClF+HF  (4)

In the case of containing 1,1,1,2-tetrafluoroethane, a side reaction with hydrogen chloride more proceeds as shown in the following formula (5):

CF₃CH₂F+HCl→CF₃CH₂Cl+HF  (5)

Therefore, after the fluorination step of (1), the acid content containing hydrogen chloride produced is preferably removed.

The acid content is removed so as to remove unreacted hydrogen fluoride (excess hydrogen fluoride) and hydrogen chloride as a by-product. Hydrogen fluoride brings about no adverse effect in the direct fluorination reaction step but hydrogen chloride is preferably removed because this product sometimes causes an adverse effect such as production of a chlorine-containing compound or chlorine fluoride as shown in the formula (4) or (5). The step of removing the acid content is performed before the direct fluorination reaction step. Examples of the method for removing the acid content includes:

(1) in the case of containing a large amount of unreacted hydrogen fluoride, a method of introducing an effluent containing the acid content into a distillation tower, extracting hydrogen chloride from the top and extracting organic substance and hydrogen fluoride from the bottom,

(2) a method of contacting the hydrogen chloride produced and unreacted hydrogen fluoride with a purifying agent, and

(3) a method of washing the acid content with water or alkali water.

In the present invention, the method for removing the acid content is not particularly limited and, for example, the method of (3) may be used. The alkali used therein may be an aqueous sodium hydroxide solution, an aqueous potassium hydroxide solution or the like. The absorbed hydrogen fluoride may be recovered and reused, and the gas passed through the washing solution is dehydrated using a dehydrating agent such as zeolite.

The gas mainly comprising CF₃CHF₂ passed through the acid content-removing step sometimes contains as impurities HCFC or CFC which is not completely fluorinated by the reaction with hydrogen fluoride, and in such a case, HCFC or CFC is preferably removed by distilling before the direct fluorination reaction step.

CF₃CHF₂ and main compounds which may be contained in CF₃CHF₂ are shown, together with respective boiling points in Table 1.

TABLE 1 Structural Boiling Compound Name Formula Point (° C.) Tetrafluoromethane CF₄ −128 Trifluoromethane CHF₃ −84 Hexafluoroethane CF₃CF₃ −78.1 Pentafluoroethane CF₃CHF₂ −48.5 Chloropentafluoroethane CF₃CClF₂ −38.7 2-Chloro-1,1,1,2-tetrafluoroethane CF₃CHClF −12 2-Chloro-1,1,1-trifluoroethane CF₃CH₂Cl 6.1

The gas mainly comprising CF₃CHF₂ is introduced into a distillation tower, then CF₄, CHF₃, CF₃CF₃, CF₃CHF₂ and CClF₂CF₃ as the low boiling fraction are extracted from the top of the distillation tower, and CF₃CHClF and CF₃CH₂Cl as the high boiling fraction are extracted from the bottom. The high boiling fraction extracted from the bottom is circulated into the reaction with hydrogen fluoride of the step (1). Here, the total amount of the compound having chlorine atom, which is contained in the distillate mainly comprising CF₃CHF₂ extracted from the top, is preferably 0.02 vol % or less. The distillate mainly comprising CF₃CHF₂ is used as a starting material in the direct fluorination reaction with fluorine gas.

The step (2) of reacting the gas mainly comprising CF₃CHF₂ with fluorine gas is described below.

The step (2) is performed in the presence of a diluting gas and the gas mainly comprising CF₃CHF₂ is set to a concentration lower than the explosion range. Specifically, the CF₃CHF₂ concentration at the reactor inlet is preferably set to about 6 mol % or less. The diluting gas is a gas containing at least one selected from the group consisting of tetrafluoromethane, hexafluoroethane, octafluoropropane and hydrogen fluoride, preferably a diluting gas rich in hydrogen fluoride.

The amount of fluorine gas used is, in terms of the molar ratio to CF₃CHF₂ (F₂/CF₃CHF₂), suitably in the range of 0.5 to 2, preferably in the range of 0.9 to 1.3. The reaction temperature is in the range of 250 to 500° C., preferably in the range of 350 to 450° C. If the reaction temperature exceeds 500° C., the objective CF₃CF₃ is disadvantageously cleaved to produce CF₄ and in the case of containing CClF₂CF₃ as an impurity, CClF₃ is disadvantageously produced due to cleavage of CClF₂CF₃, whereas if it is less than 250° C., the reaction slowly proceeds and this is not preferred.

The method for purifying the gas distilled out from the reaction step of (2) is not particularly limited. The remaining unreacted fluorine gas may be removed by adding, for example, trifluoromethane as HFC and then the residue is distilled to separate, for example, hydrogen fluoride and organic substance. The separated hydrogen fluoride is reused as the diluting gas in the direct fluorination reaction of the step (2) but may also be used as a starting material in the fluorination reaction of (1). The composition of the organic substance separated greatly differs depending on the diluting gas used for the reaction and in the case of using a gas rich in hydrogen fluoride or in the objective CF₃CF₃, the organic substance obtained contains CF₃CF₃ as a main component. In the case of using tetrafluoromethane or octafluoropropane as the diluting gas, the gas is purified by again performing distillation. In either case, high-purity CF₃CF₃ can be obtained by repeatedly performing the distillation operation according to the compositional ratio of the organic substance obtained.

In the distillation for purification of the organic substance, although it may vary depending on the compositional ratio, for example, an inert gas and CF₄ as the low boiling fraction are extracted from the top of the first distillation tower and the gas mainly comprising CF₃CF₃ is extracted from the bottom and introduced into the second distillation tower. Then, an inert gas and trifluoromethane as the low boiling fraction are extracted from the top of the second distillation tower and the gas mainly comprising CF₃CF₃ is extracted from the bottom and introduced into the third distillation tower to extract high-purity CF₃CF₃ from the top, thereby performing the purification. The gas containing CClF₂CF₃ collected from the bottom in the third distillation may be circulated into the reaction step with hydrogen fluoride of (1).

The thus-purified CF₃CF₃ contains almost no impurities and high-purity CF₃CF₃ can be obtained. The purity thereof is 99.9997 vol % or more, and 1 volppm or less of the compound having chlorine atom and 1 volppm or less of pentafluoroethane are contained as impurities.

As the analysis method of CF₃CF₃ having a purity of 99.9997 vol % or more, gas chromatography (GC) using TCD method, FID method (each including the precut method) or ECD method, or an instrument such as gas chromatography mass spectrometer (GC-MS) may be used.

Use of CF₃CF₃ obtained by the production process of the present invention is described below.

The high-purity CF₃CF₃ can be used as an etching gas at the etching step in the process of manufacturing a semiconductor device and also can be used as a cleaning gas at the cleaning step in the process of manufacturing a semiconductor device.

In the process of manufacturing a semiconductor device such as LSI and TFT, a thin or thick film is formed using CVD, sputtering or vapor deposition, and the film is etched to form a circuit pattern. In the apparatus for forming a thin or thick film, cleaning for removing unnecessary deposits accumulated on the inner wall of the apparatus, jigs and the like is performed, because the produced unnecessary deposits cause generation of particles and must be removed on occasions so as to produce a film having good quality.

The etching process using CF₃CF₃ can be performed under various dry etching conditions such as plasma etching and microwave etching, and CF₃CF₃ may be used by mixing it with an inert gas such as He, N₂ and Ar or with a gas such as HCl, O₂ and H₂ at an appropriate ratio.

EXAMPLES

The present invention is described in greater detail below by referring to the Examples and Comparative Examples, however, the present invention is not limited to these Examples.

Raw Material Example 1

In the presence of a fluorination catalyst, tetrachloroethylene (CCl₂═CCl₂) was reacted with HF at a reaction pressure of 0.4 MPa, a reaction temperature of 300° C. and a molar ratio HF/tetrachloroethylene of 4 (first reaction) and then, the reaction was further continued at a reaction pressure of 0.4 MPa, a reaction temperature of 330° C. and a molar ratio HF/intermediate (CF₃CHCl₂+CF₃CHClF) of 4 (second reaction). After the reaction, the removal of acid content and a distillation operation were performed by a conventional method, and the distillate was analyzed by gas chromatography, as a result, crude CF₃CHF₂ (Raw Material 1 of CF₃CHF₂) having a composition shown in Table 2 was obtained.

TABLE 2 Compound Purity (vol %) CF₃CHF₂ 99.4513 CH₃Cl 0.0011 CHClF₂ 0.0008 CHF₃ 0.0224 CClF₃ 0.0005 CF₃CClF₂ 0.5216 CF₃CHClF 0.0008 CF₃CCl₂F 0.0009 CF₃CH₂Cl 0.0006

Raw Material Example 2

Raw Material 1 of CF₃CHF₂ obtained by the above-described method was repeatedly distilled by a conventional method, and the distillate was analyzed by gas chromatography, as a result, crude CF₃CHF₂ (Raw Material 2 of CF₃CHF₂) having a composition shown in Table 3 was obtained.

TABLE 3 Compound Purity (vol %) CF₃CHF₂ 99.8000 CHClF₂ 0.0002 CHF₃ 0.0038 CF₃CClF₂ 0.1960

Catalyst Example 1

Into a 10 L-volume container containing 0.6 L of pure water, a solution containing 452 g of Cr(NO₃)₃.9H₂O dissolved in 1.2 L of pure water and 0.31 L of 28% aqueous ammonia were added dropwise over about 1 hour while stirring under the control to give a reaction solution having a pH of 7.5 to 8.5. The resulting hydroxide slurry was filtrated, thoroughly washed with pure water and then dried at 120° C. The thus-obtained solid was pulverized, mixed with graphite and then pelletized by a tabletting machine. The pellets obtained were calcined at 400° C. for 4 hours in a nitrogen stream to obtain a catalyst precursor. This catalyst precursor was filled into an Inconel-made reactor and subsequently subjected to a fluorination treatment (activation of catalyst) at an atmospheric pressure and 350° C. in an atmosphere of HF diluted with nitrogen, then in a 100% HF stream, and further at 450° C. in an atmosphere of HF diluted with nitrogen to prepare a catalyst.

Catalyst Example 2

Into a 10 L-volume container containing 0.6 L of pure water, a solution containing 452 g of Cr(NO₃)₃.9H₂O and 42 g of In(NO₃)₃.nH₂O (n is about 5) dissolved in 1.2 L of pure water, and 0.31 L of 28% aqueous ammonia were added dropwise over about 1 hour while stirring under the control of respective flow rates of two aqueous solutions to give a reaction solution having a pH of 7.5 to 8.5. The resulting hydroxide slurry was filtrated, thoroughly washed with pure water and then dried at 120° C. for 12 hours. The thus-obtained solid was pulverized, mixed with graphite and then pelletized by a tabletting machine. The pellets obtained were calcined at 400° C. for 4 hours in a nitrogen stream to obtain a catalyst precursor. Into an Inconel-made reactor, the catalyst precursor was filled and subsequently subjected to a fluorination treatment (activation of catalyst) in the same manner as in Catalyst Example 1 to prepare a catalyst.

Example 1

Step (1)

Into an Inconel 600-type reactor having an inner diameter of 1 inch and a length of 1 m, 150 ml of the catalyst prepared in [Catalyst Example 1] was filled, and the temperature was elevated to 440° C. while passing nitrogen. Thereto, hydrogen fluoride was fed at 3.5 NL/hr and then Raw Material 1 of CF₃CHF₂ obtained in [Raw Material Example 1] was fed at 3.5 NL/hr. The feeding of nitrogen gas was stopped and the reaction was initiated. After 2 hours, the exhaust gas was washed with an aqueous potassium hydroxide solution to remove the acid content and thereafter, the gas composition was analyzed by gas chromatography, as a result, a gas having a composition shown in Table 4 was obtained.

TABLE 4 Compound Purity (vol %) CF₃CHF₂ 99.3273 CF₄ 0.0113 CHF₃ 0.0215 CF₃CF₃ 0.6120 CF₃CClF₂ 0.0156 CF₃CHClF 0.0112 CF₃CH₂Cl 0.0011

Example 2

Step (1)

A reaction and an analysis were performed under the same conditions through the same operations as in Example 1 except for filling 150 ml of the catalyst prepared in Catalyst Example 2 as the catalyst. The analysis results are shown in Table 5.

TABLE 5 Compound Purity (vol %) CF₃CHF₂ 99.2732 CF₄ 0.0170 CHF₃ 0.0212 CF₃CF₃ 0.6720 CF₃CClF₂ 0.0068 CF₃CHClF 0.0098 CF₃CH₂Cl 0.0015

As is apparent from the analysis results shown in Table 5, when a fluorination catalyst obtained by adding indium to chromium is used, the conversion ratio of CClF₂CF₃ to CF₃CF₃ is improved.

Example 3

Step (1)

A reaction and an analysis were performed under the same conditions through the same operations as in Example 1 except for changing the reaction temperature to 300° C. The analysis results are shown in Table 6.

TABLE 6 Compound Purity (vol %) CF₃CHF₂ 99.4314 CF₄ 0.0023 CHF₃ 0.0221 CF₃CF₃ 0.0387 CF₃CClF₂ 0.4829 CF₃CHClF 0.0014 CF₃CH₂Cl 0.0005

Example 4

Step (1)

A reaction and an analysis were performed under the same conditions through the same operations as in Example 1 except for changing the reaction temperature to 500° C. The analysis results are shown in Table 7.

TABLE 7 Compound Purity (vol %) CF₃CHF₂ 99.1948 CF₄ 0.1488 CHF₃ 0.0168 CF₃CF₃ 0.5880 CHClF₂ 0.0069 CF₃CClF₂ 0.0148 CF₃CHClF 0.0256 CF₃CCl₂F 0.0021 CF₃CH₂Cl 0.0022

Example 5

Step (1)+Step (2)

Into an Inconel 600-type reactor having an inner diameter of 1 inch and a length of 2 m, 150 ml of the catalyst prepared in [Catalyst Example 2] was filled, and the temperature was elevated to 430° C. while passing nitrogen. Thereto, hydrogen fluoride was fed at 5.0 NL/hr and then Raw Material 2 of CF₃CHF₂ obtained in [Raw Material Example 2] was fed at 8.0 NL/hr. Subsequently, the feeding of nitrogen gas was stopped and 2 hours after the initiation of the reaction, the exhaust gas was washed with aqueous potassium hydroxide solution to remove the acid content. The resulting gas composition was analyzed by gas chromatography, as a result, a gas having the composition shown in Table 8 was obtained.

TABLE 8 Compound Purity (vol %) CF₃CHF₂ 99.7922 CF₄ 0.0018 CHF₃ 0.0036 CF₃CF₃ 0.1980 CF₃CClF₂ 0.0008 CF₃CHClF 0.0036

The gas having the composition shown in Table 8 after the removal of the acid content was collected under cooling and purified by distillation according to a conventional method. The gas obtained after the purification was analyzed and the results are shown in Table 9.

TABLE 9 Compound Purity (vol %) CF₃CHF₂ 99.7950 CF₄ 0.0019 CHF₃ 0.0035 CF₃CF₃ 0.1988 CF₃CClF₂ 0.0008

As is apparent from the analysis results shown in Table 9, by performing distillation, chlorotetrafluoroethane can be mostly removed.

Using the gas mainly comprising CF₃CHF₂ after the purification by distillation obtained above, a direct fluorination reaction with fluorine gas was performed.

An Inconel 600-type reactor having an inner diameter of 20.6 mmφ and a length of 500 mm (using a heating system by an electric heater; the reactor had been subjected to a passivation treatment with fluorine gas at a temperature of 500° C.) was heated to a temperature of 420° C. while passing nitrogen gas at 30 NL/hr.

Then, hydrogen fluoride was fed at 50 NL/hr, and into one gas flow diverged from the diluting gas, the gas mainly comprising CF₃CHF₂ was fed at 3.5 NL/hr. Thereafter, fluorine gas was similarly fed at 3.85 NL/h to another gas flow diverged from the diluting gas to perform a reaction. After 3 hours, the reaction product gas was washed with an aqueous potassium hydroxide solution and an aqueous potassium iodide solution to remove hydrogen fluoride and unreacted fluorine gas. Subsequently, the gas composition was analyzed by gas chromatography. The analysis results are shown in Table 10.

TABLE 10 Compound Purity (vol %) CF₃CHF₂ 0.0001 CF₄ 0.0456 CF₃CF₃ 99.9536 CF₃CClF₂ 0.0007

The gas after the removal of the acid content was collected under cooling and purified by distillation.

The gas after the purification was analyzed by gas chromatography using TCD method, FID method, ECD method and GC-MS method, and the analysis results are shown in Table 11.

TABLE 11 Compound Purity (vol %) CF₃CHF₂  0.9 vol ppm CF₄ <0.4 vol ppm SF₆ <0.4 vol ppm CF₃CClF₂ <0.1 vol ppm CF₃CF₂ 99.9998 vol %

As is apparent from the analysis results shown in Table 11, CF₃CF₃ after the purification contains almost no other impurities, thus, high-purity CF₃CF₃ is obtained and the purity thereof is 99.9997 vol % or more.

Comparative Example 1

An Inconel 600-type reactor having an inner diameter of 20.6 mmφ and a length of 500 mm (using a heating system by an electric heater; the reactor had been subjected to a passivation treatment with fluorine gas at a temperature of 500° C.) was heated to a temperature of 420° C. while passing nitrogen gas at 30 NL/h.

Then, hydrogen fluoride was fed at 50 NL/hr, and into one gas flow diverged from the diluting gas, Raw Material 1 of CF₃CHF₂ obtained in (Raw Material Example 1] was fed at 3.5 NL/hr. Thereafter, fluorine gas was similarly fed at 3.85 NL/h into another gas flow diverged from the diluting gas to perform a reaction. After 3 hours, the reaction product gas was washed with an aqueous potassium hydroxide solution and an aqueous potassium iodide solution to remove hydrogen fluoride and unreacted fluorine gas. Subsequently, the gas composition was analyzed by gas chromatography. The analysis results are shown in Table 12.

TABLE 12 Compound Purity (vol %) CF₃CHF₂ 0.0003 CF₄ 0.0568 CClF₃ 0.0036 CF₃CF₃ 99.4160 CF₃CClF₂ 0.5233

As is apparent from the analysis results shown in Table 12, when CF₃CHF₂ containing a compound having chlorine atom within the molecule as impurities is reacted with fluorine gas, CClF₃ (chlorotrifluoromethane) which is a substance difficult to separate, is produced.

Then, the gas having the composition shown in Table 12 after the removal of the acid content was collected under cooling and purified by distillation. The gas obtained after the purification was analyzed and the results are shown in Table 13.

TABLE 13 Compound Purity (vol %) CF₃CHF₂ 0.0003 CF₄ <0.0001 CClF₃ 0.0036 CF₃CF₃ 99.9959 CF₃CClF₂ <0.0001

As is apparent from the analysis results shown in Table 13, CClF₃ is a compound hard to separate.

EFFECTS OF THE INVENTION

As described in the foregoings, by using starting gas mixture containing CF₃CHF₂ and a compound having chlorine atom, high-purity CF₃CF₃ can be produced, and the high-purity CF₃CF₃ produced according to the present invention can be used as an etching gas or a cleaning gas in the process of manufacturing a semiconductor device. 

What is claimed is:
 1. A process for producing hexafluoroethane, comprising the following two steps: (1) a step of reacting a gas mixture containing pentafluoroethane and a compound having chlorine atom with hydrogen fluoride in the gaseous phase in the presence of a fluorination catalyst to fluorinate said compound having chlorine atom; and (2) a step of reacting the gas mixture containing pentafluoroethane and the fluorinated compound obtained in said step (1) with a fluorine gas in the gaseous phase in the presence of a diluting gas.
 2. The process for producing hexafluoroethane as claimed in claim 1, wherein said compound having chlorine atom is at least one compound selected from the group consisting of chloromethane, chlorotrifluoromethane, chloropentafluoroethane, dichlorotetrafluoroethane, chlorotetrafluoroethane, chlorotrifluoroethane and chlorotrifluoroethylene.
 3. The process for producing hexafluoroethane as claimed in claim 1 or 2, wherein the total amount of the compound having chlorine atom contained in the gas mixture of the step (1) is 1 vol % or less.
 4. The process for producing hexafluoroethane as claimed in claim 1 or 2, wherein the total amount of the compound having chlorine atom contained in the gas mixture of the step (1) is 0.5 vol % or less.
 5. The process for producing hexafluoroethane as claimed in any one of claims 1 to 4, wherein in said step (1), the fluorination catalyst is a bulk catalyst obtained by adding indium to an oxide of chromium.
 6. The process for producing hexafluoroethane as claimed in any one of claims 1 to 5, wherein in said step (1), the temperature at the reaction with hydrogen fluoride in the presence of a fluorination catalyst is in the range of 150 to 480° C.
 7. The process for producing hexafluoroethane as claimed in any one of claims 1 to 6, wherein in said step (1), the molar ratio of hydrogen fluoride/organic substance contained in the gas mixture is in the range of 0.5 to
 5. 8. The process for producing hexafluoroethane as claimed in any one of claims 1 to 7, wherein a step of removing an acid content containing hydrogen chloride produced is conducted before said step (2).
 9. The process for producing hexafluoroethane as claimed in any one of claims 1 to 8, wherein a step of separating chlorotetrafluoroethane and/or chlorotrifluoroethane, and returning the chlorotetrafluoroethane and/or chlorotrifluoroethane separated to the step (1) is conducted before said step (2).
 10. The process for producing hexafluoroethane as claimed in any one of claims 1 to 9, wherein in said step (2), the total amount of the compound having chlorine atom contained in the gas mixture is 0.02 vol % or less.
 11. The process for producing hexafluoroethane as claimed in any one of claims 1 to 10, wherein in said step (2), the fluorinated compound contained in the gas mixture is mainly composed of hexafluoroethane.
 12. The process for producing hexafluoroethane as claimed in any one of claims 1 to 11, wherein in said step (2), the diluting gas is a gas containing at least one selected from the group consisting of tetrafluoromethane, hexafluoroethane, octafluoropropane and hydrogen fluoride.
 13. The process for producing hexafluoroethane as claimed in any one of claims 1 to 12, wherein in said step (2), the diluting gas is a gas rich in hydrogen fluoride.
 14. The process for producing hexafluoroethane as claimed in any one of claims 1 to 13, wherein in said step (2), the temperature at the reaction of gas mixture containing the fluorinated compound with fluorine gas is in the range of 250 to 500° C.
 15. The process for producing hexafluoroethane as claimed in any one of claims 1 to 14, wherein in said step (2), the temperature at the reaction of gas mixture containing the fluorinated compound with fluorine gas is in the range of 350 to 450° C.
 16. A hexafluoroethane product comprising hexafluoroethane having a purity of 99.9997 vol % or more.
 17. The hexafluoroethane product as claimed in claim 16, wherein the content of the compound having chlorine atom is 1 volppm or less and the content of the pentafluoroethane is 1 volppm or less.
 18. An etching gas comprising the hexafluoroethane product described in claim 16 or
 17. 19. A cleaning gas comprising the hexafluoroethane product described in claim 16 or
 17. 